In the field of automated machining, a numerically controlled machine tool positioning system (CNC) is often used in conjunction with a probe sensor to position a workpiece surface which is to be drilled, cut, welded or machined. During the probing cycle, either the workpiece or the probe is moved by the positioning system until contact between the workpiece and probe occurs. Upon contact, the probing system provides a "trip" signal to the positioning system which then records the location of the workpiece. Typically, the probe sensor is a mechanical stylus or other contact probe. The probe has to be made compliant enough to withstand accidental shocks and overtravel, yet be able to accurately return to its initial position.
Mechanical stylus probing systems present additional difficulties, particularly in laser machining operations. First, the probe must be displaced from the drilling light path to prevent destruction of the probe by the laser beam. Second, in traditional machining equipment the probe consumes a tooling position. In a laser machining system where there are no tool holders, a special positioning device would be required to accommodate the probe. Additionally, a mechanical probe requires an external, absolute reference for calibration before every probe cycle. The reference must maintain an exact relationship between itself, the part fixture and the machining equipment. This relationship must be insensitive to the environment and must be kept clean. If this reference is damaged, it must be replaced and the machine recalibrated. Finally, the probing system requires a regular maintenance schedule to maintain accuracy.
In many machine tools, including lasers used for drilling, cutting, or welding, a visible Helium Neon laser beam is used for positioning and alignment of the workpiece by an operator. In addition, many machine tools are equipped with a television camera system to view the machining operation. The present invention utilizes these existing components to automatically position a workpiece surface without physically contacting the surface. Thus, the present invention can be installed into an existing machine tool system easily and at a relatively small cost in comparison to a "contact" probing system.